1. Field of the Invention
The present invention relates to a high bypass-ratio turbofan jet engine, and more particularly to a high bypass-ratio turbofan jet engine which is capable of increasing the bypass ratio without increasing the fan diameter, and of reducing air resistance acting on the engine.
2. Description of the Related Art
In a turbofan jet engine that is widely used in aircraft flying at subsonic speeds, generally, the larger the ratio between the quantity of air passing through a core part and the quantity of air passing through a fan part (bypass ratio), the closer the exhaust velocity to the cruising speed and the higher the propulsion efficiency. Therefore, an engine with high bypass ratio tends to have improved fuel consumption and generate low jet noise. However, an engine with a larger fan has the following problems. First, the weight of moving and stationary blades and fan case/nacelle is large. Secondly, due to the large engine diameter, the main landing gears need to be made long to maintain a predetermined or more distance between the ground and the engines in the case of aircraft with the engines beneath the wings. Thirdly, when the flow rate on the outer circumference is kept lower than or equal to a predetermined flow rate in order to maintain the fan efficiency, the rotation speed of the engine of the large-diameter fan is reduced and the rotation speed of low-pressure turbines rotating coaxially with the fan is also reduced, degrading the efficiency of the low-pressure turbines. As one of the measures to improve these problems, research and development is carried out on a deceleration type turbofan jet engine (Geared Turbofan) which is provided with a speed reduction mechanism between low-pressure turbines and a fan and optimizes the rotation speed of the fan and turbines. In this turbofan jet engine, however, it is considered that, when thrust is increased to a predetermined level or higher, the impact of increase in weight of the speed reduction mechanism and its lubrication system or the loss of shaft power is more significant than the effect of improvement of fuel consumption.
Although an unducted fan without a nacelle and an advanced turbo prop have been studied as a way to increase the bypass ratio while keeping the fan diameter, when a fan blade is damaged for any reason, there arise problems of noise and difficulty in securing the airworthiness for preventing fragments of the fan blade from colliding with the aircraft body and damaging it.
When increasing the fan diameter and bypass ratio as described above, the increase in weight of the fan case/nacelle, the enlargement of the main landing gears, and the degradation of the efficiency of the low-pressure turbines bring about significantly negative effects.